The invention relates to a circuit and a process for regulating the speed of universal electric motors, particularly in portable power tools, to at least one additional, lower speed.
An additional lower speed, for a hand-held power tool, is normally achieved by a phase angle-controlled voltage control used to reduce the effective voltage in a series-wound electric motor. Thus, the shift of the speed-torque characteristic curve of the motor, which is dependent on the effective motor voltage as a parameter, causes the speed to drop through the torque. The drawback of this simple regulation process is its great load-dependency at low speeds.
According to EP 594,907, an electric motor having a different characteristic curve is operated at a high speed and a low speed, whereby the minimum speed is limited by a secondary impedance of the magnet coil that is triggered by the voltage of the magnet coil through a circuit formed by individual Z-diodes and a power switch. The speed, however, still remains load-dependent.
According to DE 19940409, an electric motor having an additional, lower speed stage is regulated to a constant speed by a voltage regulation that is phase angle-controlled through the torque on the basis of the characteristic curve of the motor, which is dependent on the effective motor voltage as a parameter. The higher torque that is consequently possible at lower speeds, however, can lie above a thermal limit torque in an electric motor that is configured for high speeds in terms of its thermal properties and its design.
According to DE 3612193, in an electric motor, in a hand-held power tool having speed regulation, when a switchover torque before the limit torque is exceeded, the regulation at the momentary effective motor voltage is switched off so that the speed drops as a function of the momentary characteristic curve of the motor in a way that is clearly perceptible to the user.
Moreover, according to DE 19609986 C1, an electric motor, in a hand-held power tool, is temporarily interrupted with drive pauses when a limit torque is exceeded until the user switches it off and, during these pauses, the hand-held power tool can only partially be used as intended.
The objective of the invention is to achieve a speed regulation for a lower speed until a limit torque is reached.
In accordance with the invention, an electric motor, preferably a series-wound electric motor, that is connected via a circuit to a source of current for purposes of speed regulation has at least one additional, lower regulated speed range, whereby in a first prioritized step, depending on the preselected speed, the regulation of the speed to a lower effective motor voltage has an upper limit, and in a second step, depending on the torque needed for the regulation to this constant speed, the effective motor voltage is regulated within the limited range.
Due to the limitation of the effective motor voltage to a value that is less than the effective operating voltage of the source of current, the entire family of the characteristic curves of the motor that are parameterized via the effective motor voltage are no longer available for the regulation of the electric motor. Consequently, critical characteristic motor lines that cause the limit torque to be exceeded, in the low speed range, are not available for the regulation of the speed to the low speed.
Preferably, the effective motor voltage is generated from the operating voltage of the source of current through a conventional phase angle control or phase sector control.
Preferably, in the phase angle control or phase sector control, the effective motor voltage has an upper limit due to another control input for the minimum phase angle, as a result of which this barrier can be simply integrated into a conventional phase angle control or phase sector control.
Preferably, when a switchover torque is reached before the limit torque, an even lower speed is preselected by the circuit, in a third step, and the process is continued with the first step, as a result of which, when the switchover torque is reached again, the electric motor is operated in discrete steps, each at a lower, constantly regulated speed. Accordingly, a hand-held power tool driven by this electric motor can still be fully used as intended.
Preferably, at a standstill with a speed of zero, the circuit generates a low effective motor voltage, which is continuously permissible for generating a residual torque when the electric motor is at rest and which is preferable for starting up again or, as an alternative, the effective current flow is interrupted, as a result of which it is reliably switched off.